The present disclosure relates generally to implantable medical devices, and, more particularly, to implantable medical devices intended for use in monitoring a patient's heart rhythm.
Implantable pacemakers and cardioverters monitor the heart's rhythm in order to detect arrhythmias and deliver appropriate therapies to terminate detected arrhythmias. In conjunction with this function, the ability of the device is to store information with regard to monitored heart rhythms has dramatically increased over the past two years. Examples of implantable pacemakers and defibrillators which have the capability of storing information related to monitor heart rhythms include U.S. Pat. No. 4,223,678 issued to Langer et al., U.S. Pat. No. 5,722,999 issued to Snell, U.S. Pat. No. 5,513,645 issued to Jacobsen et al. and U.S. Pat. No. 5,312,446 issued to Holschbach et al. In addition, there are subcutaneously implantable monitoring devices which do not deliver any anti-arrhythmia therapies to the heart but simply store information regarding a patient's heart rhythms for later uplink to an external device. Such devices are disclosed in U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 5,135,004 issued to Adams and U.S. Pat. No. 5,497,780 issued to Zehender.
Data stored relating to a patient's heart rhythm may include information relating to heart rate trends over time, as disclosed in U.S. Pat. No. 5,088,488 issued to Markowitz et al., U.S. Pat. No. 5,330,513 issued to Nichols et al. and U.S. Pat. No. 5,603,331 issued to Heemels et al., as well as information relating to heart rate variability over time, as disclosed in U.S. Pat. No. 5,749,900 issued to Schroeppel et. al., U.S. Pat. No. 5,466,245 issued to Spinelli et al., U.S. Pat. No. 5,411,131 issued to Yomtov et al. and U.S. Pat. No. 5,437,285 issued to Verrier et al.
Typically, measurements of heart rate variability in such devices is accomplished by employing one of the standard measurements of heart rate variability such as the standard deviation of all normal-to-normal (SDNN) or standard deviation of average normal-to-normal (SDANN) indices, described in U.S. Pat. No. 5,749,900 issued to Schroeppel et al. and U.S. Pat. No. 5,411,031 issued to Yomtov, cited above. Another method to measure heart rate variability is described in U.S. Pat. No. 6,508,771 issued to Padmanabhan et al. in which median atrial interval value is measured every five minutes and then the standard deviation of these intervals is computed over a certain period of time.
Heart rate variability can be useful for determining cardiac sympathetic reserve, which can be helpful in determining whether a patient may benefit from cardiac therapies (e.g. cardiac resynchronization therapy (CRT)). Patients responsive to CRT typically exhibit a baseline sympathetic uptake function that indicates a preserved sympathetic function. In contrast, patients that exhibit severely impaired baseline sympathetic function are less likely to benefit from CRT. Present methods that calculate heart rate variability can be less accurate resulting in some patients not receiving implantable medical devices when in fact the patient could benefit from a cardiac therapy. For example, conventional devices determine heart rate variability by measuring a median atrial interval value every five minutes and then computing the standard deviation of these intervals over a 24 hour period. The conventional method can occasionally incorrectly determine that a patient's heart rate variability is decreasing during a period of activity. It is therefore desirable to develop a more accurate method to determine heart rate variability in order to estimate cardiac sympathetic reserve, which, in turn, whether a patient can benefit from cardiac therapy delivered through an implantable medical device.